Even with the rising popularity of mobile devices, many of these devices are not truly wireless. Instead, many devices remain tethered to power cords—at least during the time that the mobile devices have their batteries re-charged. For some devices, energy harvesting techniques may be integrated to facilitate totally wireless operation. Often, energy harvesting devices are opportunistic, harvesting from whatever sources are available.
One popular energy harvesting technique involves radio-frequency (RF) sources such as wireless access points or dedicated base stations or power transmitters. RF energy sources include ambient sources and intentionally broadcast sources. Ambient RF energy can be obtained from mobile and WiFi networks; that is, nearby mobile phones, WiFi routers and devices, and base stations or broadcast radio towers generate ambient RF energy. Intentional RF energy sources can be operated in a manner that keeps energy storage devices fully charged or providing power for device activation. RF signals may be intentionally broadcast on unlicensed frequency bands (or in whitespace) in a continuous, scheduled, or on-demand basis. Intentional sources can be deployed in a network similar to WiFi routers or base stations where multiple transmitters provide coverage over a wide area. Current examples of intentional sources include RFID readers.